1. Field of the Invention
The present invention relates to a USB power supply.
2. Description of the Related Art
Battery-driven devices such as cellular phone terminals, tablet terminals, laptop personal computers, and portable audio players each include a rechargeable secondary battery and a charger circuit that charges the secondary battery as built-in components. A charger circuit is known, which charges a secondary battery using a DC voltage supplied from a USB (Universal Serial Bus) host adapter via a USB cable.
At present, charger circuits mounted on mobile devices conform to a specification which is referred to as the “USB Battery Charging Specification” (which will be referred to as the “BC specification” hereafter). There are several kinds of host adapters. As the kinds of chargers that conform to revision 1.2 of the BC specification, SDP (Standard Downstream Port), DCP (Dedicated Charging Port), and CDP (Charging Downstream Port) have been defined. The current (current capacity) that can be provided by a host adapter is defined according to the kind of charger. Specifically, DCP and CDP are defined to provide a current capacity of 1500 mA. Also, SDP is defined to provide a current capacity of 100 mA, 500 mA, or 900 mA, according to the USB version.
As a next-generation secondary battery charging method or system using USB, a specification which is referred to as the “USB Power Delivery Specification” (which will be referred to as the “PD specification” hereafter) has been developed. The PD specification allows the available power to be dramatically increased up to a maximum of 100 W, as compared with the BC specification, which provides a power capacity of 7.5 W. Specifically, the PD specification allows a USB bus voltage that is higher than 5 V (specifically, 12 V, 20 V). Furthermore, the PD specification allows a charging current that is greater than that defined by the BC specification (specifically, the PD specification allows a charging current of 2 A, 3 A, 5 A).
1. With the PD specification, a power supply apparatus, which functions as a power supply source, generates a bus voltage VBUS that is switchable between multiple voltages. Description will be made below regarding an example in which the power supply apparatus is capable of switching the bus voltage VBUS between 5 V and 20 V. FIG. 1 is a circuit diagram showing a power supply apparatus 300 investigated by the present inventor.
The power supply apparatus 300 includes a DC/DC converter 302, resistors R1 and R2, a control IC (Integrated Circuit) 304, a bus line 306, and a connector 308. The connector 308 is coupled to an unshown power receiving apparatus via a USB cable 310.
The DC/DC converter 302 includes an OUT (output) terminal and an FB (feedback) terminal. The OUT terminal is coupled to the bus line 306. The DC/DC converter 302 stabilizes the output voltage VBUS such that a feedback voltage VFB fed back to the FB terminal matches a predetermined reference voltage VREF.
The resistors R1 and R2 are arranged in series between the bus line 306 and the ground, so as to divide the bus voltage VBUS, thereby generating the feedback voltage VFB that is proportional to the bus voltage VBUS. With the voltage dividing ratio as K, the following Expression (1) holds true. The resistor R2 is configured as a variable resistor, thereby allowing the voltage dividing ratio K to be adjusted.VFB=VBUS×K  (1)
As described above, as a result of the feedback operation, the relation VFB=VREF holds true. Accordingly, the bus voltage VBUS is stabilized to the voltage level represented by the following Expression (2).VBUS=VREF/K  (2)
The control IC 304 transmits/receives a signal to/from an unshown power receiving apparatus via the bus line 306. The control IC 304 determines the voltage level of the bus voltage VBUS based on negotiation. Subsequently, the voltage dividing ratio K is switched according to the voltage level thus determined, thereby allowing the bus voltage VBUS to be generated according to a request from the power receiving apparatus.
With the power supply apparatus 300 shown in FIG. 1, the single DC/DC converter 302 supports the generation of a bus voltage VBUS that is switchable between multiple different voltage levels. This leads to a problem of increased costs required for the DC/DC converter 302.
2. It is conceivable that, in some cases, the impedance of the charging path rises due to partial disconnection of a charging cable, loose connection of a connector, or the like. The PD specification supports the flow of a large amount of current on the order of several A, which provides a rapid charging operation. However, in a case in which such a large amount of current flows through a charging path having high impedance, this leads to a problem of heat generation or the like.
The conventional BC specification supports a constant bus voltage VBUS of 5 V. Accordingly, by setting a threshold voltage VTH that is lower than the constant bus voltage, such an arrangement is capable of detecting an abnormal state of the system. This function is also referred to as “UVLO (Under Voltage Lock Out)”, and is widely employed in various kinds of devices. However, with the PD specification, the bus voltage VBUS is switchable between various voltage levels according to the kind of device. Accordingly, such a threshold voltage VTH cannot be determined. Thus, it is difficult for such a conventional technique to detect an abnormal state.